"...How was it 'quite the experience'? Did it accelerate like mad? I'm intrigued by rosin in soap myself. ..."
You should try it, Ann. And then we need to design a merit badge for the experience.
The old US and British soapmaker's methods for making rosin soap were my inspiration. They mostly used tallow sometimes with some coconut, depending on how old the recipe is. Older ones would use pretty much all tallow because (so I gather) coconut oil in that day was hard to get and it was often disgustingly smelly.
They used two main methods of making rosin soap. The first method was to saponify the rosin separately and add that to a regular type of soap. The other way was to start the regular soap then add rosin during saponification when the temperatures were high enough to melt the rosin. Because the way they made "boiled" soap is different than the way we make soap, I wasn't quite sure which method would work best. The advantage of the first method to the old boys was the rosin soap did not get as dark so the finished soap was prettier, but the method was more time consuming and required two sets of soap kettles and related equipment. The second method was simpler and eventually became the main method used.
Problem is, I don't have any tallow so I couldn't use the old recipes directly. But I do soap with lard, so I came up with a "tallow dupe" that uses lard, coconut, and stearic acid to approximate the fatty acid composition of tallow. To 65% of my "tallow", I added olive oil at 25% for mildness because folks talk about rosin soap being harsh, and I set the rosin at 10% -- kind of a middle ground between "a dab" and "a lot", based on the recipes I'd seen, both old and contemporary. I used 2% superfat and a lye solution concentration of 30% (a little wetter than I usually use, but not "full water").
I hesitated to jump into doing this soap, because I knew stearic and rosin are messy to melt and they saponify really quickly. I have experience with stearic acid because I use it in my lotions and shave soaps, but the rosin would be a new experience for me.
Plan A was to (a) melt the rosin and stearic together, (b) saponify the fats like a normal soap recipe using the lye they needed for saponification, (c) saponify the stearic-rosin separately with its own lye, (d) add the stearic-rosin to the fats when the fats were at trace, and (e) see what happened from there. I wasn't sure if the whole thing would be fluid at step (e) or if I'd have to HP the resulting mix or who-knows-what, so I planned to be flexible.
All went okay until I made the newbie mistake of adding the lye to the rosin-stearic too quickly. I learned rosin and stearic acid will saponify as fast as a greased pig in a pig-wrangling contest and they generate plenty of heat in the process! I hadn't seen quite this aggressive behavior with just stearic, so I suspect the rosin was the main culprit. As I was cleaning up the resulting volcano mess, I pondered what my Plan B was going to be.
I decided Plan B was to (a) melt the rosin and stearic together, (b) saponify the fats using ALL of the lye needed for the entire recipe, (c) add the stearic-rosin to the fats when the fats were at trace, (d) stir like mad, and (e) see what happened from there. This worked better, but it was still quite the rodeo. The stearic-rosin saponified quickly once added to the soap batter -- about as fast as stearic alone saponifies, but with a LOT more heat being generated. The batter quickly thickened to a cold mashed potatoes consistency, then just as quickly hit reverse gear and squealed into full gel.
As I kept stirring, I could feel the heat radiating onto my hands and wrists, and I could see thin crusts of batter were drying out around the edge of the bowl from the heat, making ugly dried clumps. After a short bit, it looked like the saponification reaction was settling down, which meant the volcano risk was dropping fast. More stirring was just going to make matters worse, so I filled my mold with the gelled soap and tamped it down as smooth as possible. I'd guess it took well under 5 minutes to get to this point from the time I added the stearic-resin to the soap pot.
The fight had gone out of the soap by the time it was molded, so I put it into a cold oven to sit overnight -- I do this to keep curious kitten noses out of trouble. The soap behaved itself quite nicely and was plenty firm enough to cut this morning. It looks better than I thought it would. It seems slightly lye heavy this morning, but patience will cure that ill. I didn't use it in the shower for obvious reasons, but it lathers fine at the sink and doesn't seem unusually drying or irritating. The bar is pleasantly hard but not brittle -- I was concerned it might be soft from what I'd read, but that's not a problem with this recipe.
Thoughts:
I think Plan A would have worked as long as the lye is added to the stearic-rosin very, very slowly with vigorous stirring the entire time. I'm not sure it has any advantage to Plan B, however.
I think I will do Plan B if I make this soap again, but I will cover the soap pot with a clear cover once the soap goes into gel ... and watch it like a hawk until the heating settles down. If the soap threatens to volcano, I'll give it a vigorous stir and put the cover back on. With any luck, this will let the saponification happen without drying out the soap. When the soap seems to be settling down, put into the mold.
Oh, and melt the stearic and rosin in a container that can be discarded after use -- a clean tin can, for example. It's possible to clean the stearic and rosin residues in a regular container, but it takes a lot of soaking and scrubbing.
I don't see how this can be done as a typical CP method -- it's more like HP without needing to add any external heat.
